In the past, in a wireless LAN system, two types of access point scan methods, passive scanning and active scanning, have been set as standard (Non-patent Document 1).
A station performing passive scanning described in “11.1.3.1 Passive scanning” of IEEE 802.11 Std. receives a beacon (a management frame) transmitted by an access point, and, based on a connection configuration included in the beacon, determines an access point to which the station is to be connected. That is, based on the connection configuration included in the received beacon, the station selects an access point whose service set identifier (SSID) coincides with the SSID of an access point registered as a candidate for connection in advance, and performs connection processing. When there are a plurality of available radio channels, the station sequentially performs scanning on the radio channels.
A station performing active scanning described in “11.1.3.2 Active scanning” of IEEE 802.11 Std. transmits a probe request for an access point scan for each of the available radio channels. The probe request is broadcasted, and, as the contents thereof, the SSID of an access point registered as a candidate for connection in advance or a broadcast SSID (SSID=0) is set. After transmitting the probe request, the station starts a timer and monitors a response, and, when the station receives no probe response within the monitoring time, the station scans for the next radio channel. On the other hand, the access point which receives the probe request transmits a probe response to a transmission source of the probe request as a response thereto when the SSID set in the probe request coincides with the SSID of the access point or is the broadcast SSID. The connection configuration included in the probe response is equal to that included in the beacon. Based on the connection configuration, the station selects an access point which coincides with the SSID of the access point registered as a candidate for connection in advance, and performs connection processing.
Non-patent Document 1: IEEE 802.11 Std. “11.1.3.1 Passive scanning”, “11.1.3.2 Active scanning”